The present invention relates to a damping force control type hydraulic shock absorber which is mounted on a suspension apparatus of a vehicle, such as an automobile.
As an example of a hydraulic shock absorber which is mounted on a suspension apparatus of a vehicle, such as an automobile, there can be mentioned a damping force control type hydraulic shock absorber which is capable of controlling a damping force in response to variations in road conditions, vehicle running conditions, etc., with the aim of improving ride quality or steering stability of a vehicle.
Such a damping force control type hydraulic shock absorber generally comprises: a cylinder in which a hydraulic fluid is sealably contained; a piston silidably provided in the cylinder so as to divide the interior of the cylinder into two chambers; and a piston rod connected to the piston. The piston includes a main fluid passage and a bypass passage for enabling communication between the two chambers in the cylinder. A damping force generating mechanism including an orifice and a disk valve is provided in the main fluid passage. A damping force control valve is provided in the bypass passage so as to adjust a flow path area of the bypass passage.
When the bypass passage is opened by virtue of the damping force control valve, the flow resistance of the hydraulic fluid between the two chambers in the cylinder is reduced, thereby generating a small damping force. On the other hand, when the bypass passage is closed, the flow resistance of the hydraulic fluid between the two chambers in the cylinder is increased, thereby generating a large damping force. Thus, damping force characteristics can be appropriately controlled by opening and closing the damping force control valve.
In this hydraulic shock absorber in which the damping force is controlled by adjusting the flow path area of the bypass passage, when the piston is operating at low speed, a damping force is generated depending on the restriction of an orifice in the fluid passage, so as to enable damping force characteristics to be widely varied. However, when the piston is operating at a medium or high speed, a damping force is dependent on the degree of opening of the damping force generating mechanism (such as the disk valve) in the main fluid passage, with the result that it is difficult to widely vary damping force characteristics.
As a countermeasure, in the damping force control type hydraulic shock absorber of Unexamined Japanese Patent Application Public Disclosure (Kokai) No. 7-332425, a pressure chamber (a pilot chamber) is formed on a back side of a disk valve which provides a damping force generating mechanism in a main fluid passage commonly used for an extension stroke and a compression stroke of the piston rod. This pressure chamber is communicated through a fixed orifice with a cylinder chamber disposed upstream of the disk valve, and communicated through a variable orifice (a flow rate control valve) with a cylinder chamber disposed downstream of the disk valve.
In this damping force control type hydraulic shock absorber, while the flow path area of the communication passage between the two cylinder chambers is adjusted by opening and closing the variable orifice, the initial pressure for opening the disk valve is varied in response to a change in pressure in the pressure chamber, which change occurs as a result of a pressure loss generated in the variable orifice. Thus, orifice characteristics (of a damping force varying substantially in proportion to the square of the piston speed) and valve characteristics (of a damping force varying substantially in proportion to the piston speed) can be controlled, thus enabling control of damping force characteristics over a wide range.
However, the damping force control type hydraulic shock absorber in Kokai No. 7-332425 involves the following problems. Namely, because the damping force is controlled by virtue of flow control using a variable orifice, the damping force which is actually generated varies, depending on a piston speed. Therefore, when a large vibration is transmitted suddenly when the vehicle encounters a bump on a road surface, the damping force sharply increases in accordance with an increase in piston speed, thus transmitting shock to the vehicle body and causing a deterioration in ride quality. Further, because the variable orifice has a small flow path area (generally only several mm2), the flow resistance of the hydraulic fluid is likely to vary due to dimensional tolerances of valve members such as a sleeve and spool, thus making it difficult to obtain stable damping force characteristics. Further, the flow resistance generated by the variable orifice largely varies depending on the viscosity of the hydraulic fluid. Therefore, damping force characteristics are greatly influenced by temperature changes, thus making it difficult to obtain stable damping force characteristics. the variable orifice has a small flow path area (generally only several mm2), the flow resistance of the hydraulic fluid is likely to vary due to dimensional tolerances of valve members such as a sleeve and spool, thus making it difficult to obtain stable damping force characteristics. Further, the flow resistance generated by the variable orifice largely varies depending on the viscosity of the hydraulic fluid. Therefore, damping force characteristics are greatly influenced by temperature changes, thus making it difficult to obtain stable damping force characteristics.
In view of the above, the present invention has been made. It is an object of the present invention to provide a damping force control type hydraulic shock absorber which is capable of controlling damping force characteristics over a wide range, directly controlling a damping force regardless of a piston speed, and suppressing an effect of dimensional tolerances of valve members and temperature changes with respect to damping force characteristics, and which is also capable of readily absorbing the sudden transmission of pronounced vibrations.
According to the present invention, there is provided a damping force control type hydraulic shock absorber comprising: a cylinder in which a hydraulic fluid is sealably contained; a piston slidably provided in the cylinder; and a piston rod having one end connected to the piston and the other end extending to an outside of the cylinder. A main fluid passage and a sub fluid passage are connected to the cylinder so as to permit flow of the hydraulic fluid therethrough in accordance with a sliding movement of the piston. The damping force control type hydraulic shock absorber further comprises: a pilot type damping valve provided in the main fluid passage; and a fixed orifice and a pressure control valve provided in the sub fluid passage so that a pressure of the hydraulic fluid between the fixed orifice and the pressure control valve in the sub fluid passage is applied to the pilot type damping valve as a pilot pressure. The pressure control valve includes a valve chamber formed between a cylindrical sleeve and a slider slidably provided in the sleeve. An axial thrust is generated in the slider due to a difference between pressure-receiving areas in the valve chamber for receiving a pressure acting in a direction of the axis of the slider. A valve opening pressure is controlled in accordance with a balance between the thrust of the slider and a thrust of a solenoid.
By this arrangement, the valve opening pressure for the pilot type damping valve is directly adjusted by adjusting the valve opening pressure for the pressure control valve, by utilizing the thrust of the slider. At the same time, the pilot pressure is varied in accordance with controlled pressure of the pressure control valve, to thereby adjust the valve opening pressure for the pilot type damping valve. In this instance, the pressure control valve generates an axial thrust of the slider due to a difference between pressure-receiving areas in the valve chamber for receiving a pressure acting in a direction of the axis of the slider. The valve opening pressure is controlled in accordance with the balance between the thrust of the slider and a thrust of a solenoid, to thereby control the damping force.
According to an embodiment of the present invention, a disk valve is connected to the sleeve or the slider and the thrust is generated in the slider due to the difference between the pressure-receiving area of the disk valve and the pressure-receiving area of the slider in the valve chamber.
By this arrangement, a sharp rise in hydraulic pressure can be relieved by deflecting the disk valve.
According to another embodiment of the present invention, an extension-stroke valve chamber and a compression-stroke valve chamber are formed at opposite end portions of the slider and damping force characteristics are varied in opposite directions between an extension stroke and a compression stroke of the piston rod.
The foregoing and other objects, features and advantages of the present invention will be apparent from the following detailed description and appended claims taken in connection with the accompanying drawings.